Lec 33 pulmonary circulation Flashcards
What is role of pulmonary circulation?
- carry de-ox blood from RV to alveoli and return ox blood to LA
What is the surface area for pulmonary vessels
125 m2
huge surface area
What is the struct of pulmonary vessels?
- thin walled vessels, distensible
- can handle entire CO from RV without increase in pressure
- low pressure, low resistance
what is pressure in pulmonary vessels
5L/min
is pressure high or low in pulmonary circulation? what about resistance?
- low pressure
- low resistance
What is systolic PA pressure?
25 mmHg [compared to SBP 120 mmHg]
what is diastolic PA pressure?
10 mmHg [compared to DBP 80 mmHg]
What is equation for flow?
Flow = Pressure gradient/Resistance
What is PVR? Equation?
pulmonary vascular resistance
PVR = pressure differential between pulm artery and LA pressures / Flow
= mPAP - mLAP/CO
How is pulmonary blood flow regulated?
- by altering resistance of arterioles
- due to change in tone of arteriolar smooth muscle
What is unique about pulmonary vascular structure/regulation?
- even smaller pulmonary arterioles have smooth muscles in their walls
- all capable of vasoconstriction
What is most important mediator of vasoconstriction?
partial pressure of O2 in alveolar gas P[AO2]
What happens to pulmonary blood vessels in response to low alveolar Po2?
- vasoconstriction via arteriolar smooth muscle
What happens to non-pulmonary vascular beds in response to low PO2? what about pulmonary vascular?
non-pulmonary: dilate with low PO2
pulmonary: constrict with low PO2
What is hypoxic vasoconstriction?
- a protective mechanism
- pulmonary arteries/arteriole smooth muscles constricts/contracts in low alveolar PO2
- decrease perfusion [Q] to poorly ventilated [V] area so reduce V/Q mismatch by directing blood flow away from damaged or poorly ventilated alveoli
What 2 things cause generalized hypoxic vasoconstriction?
- diffuse lung disease
- high altitude
What is the cellular mech of hypoxic pulmonary vasoconstriction?
- at low alveolar PO2 [less than 70 mmHg]
- inhibition of smooth muscle K+ channels
- get depolarization
- opens voltage gated Ca channels
- get Ca influx -> smooth muscle contracts
What 3 things mediate hypoxic pulm vasoconstriction?
- NO [dilator]
- endothelin [constrictor]
- arachidonic acid products [PGI2, TXA2] [dilator]
What is role of NO in hypoxic vasoconstriction?
- NO inhibits hypoxic vasoconstrction
- activates cGMP –> relaxes smooth muscle
- causes vasodilation and anti-proliferation
What is nitric oxide synthase [NOS]?
- synthesizes NO from L-arginine
- affected by low PO2
What is endothilin?
- vasoconstrictor and causes proliferation
- plays role in hypoxic pulm vasoconstriction
what is role of PGI2 [prostacyclin] in hypoxic vasoconstriction?
- activates cAMP
- causes vasodilation and antiproliferation
What happens in high altitude to PO2, vasoconstriction/dilation, PVR, etc?
- reduced barometric P –> decrease in inspired PO2 –> low alveolar PO2
- get global vasoconstriction
- increased PVR
- over time causes RV pressure overload and hypertrophy
What happens in fetal circulation to PO2, PVR, etc?
- global vasoconstriction due to low alveolar PO2
- high PVR
- pulm blood flow is only 15% of CO
- at birth when first breaths, increases alveolar PO2 to 100 mmHg
What is order of size Pa [arterial], PA [alveolar], Pv [venous] in zone 1 [apex of lung]?
PA> Pa > Pv
alveolar > arterial > venous
What is order of size Pa [arterial], PA [alveolar], Pv [venous] in zone 3 [base of lung]?
Pa > Pv > PA
arterial > venous > alveolar
What is order of size Pa [arterial], PA [alveolar], Pv [venous] in zone 2 [middle of lung]?
Pa > PA > Pv
arterial > alveolar > venous
What happens in zone 1?
- gravitational effect causes lower arterial P than alveolar [atmospheric] P
- causes pulm arterioles compressed
- closure of capillaries
- low blood flow
Which zone of lung has least blood flow?
apex
What two circumstances can cause blood vessels to close off and stop flow in zone 1?
- arterial P gets lower: hemorrhage
- alveolar P gets higher: pos pressure on mechanical ventilator
What happens to zone 1 if no blood flow there?
- under-perfused
- no gas exchange
- becomes a physiologic dead space
What happens in zone 2?
- gravitational effect causes P arterial > P alveolar
- alveolar P still higher than venous P
- blood driven by arterial and alveolar pressure difference
What is difference in the pressure difference that drives blood flow between zone 2 of lung and systemic vascular beds? What about zone 3 of lung?
zone 2 of lung: P arterial - P alveolar
systemic: P arterial - P venous
zone 3 of lung: P arterial - P venous
What happens in zone 3 of lung?
- gravity increases P arterial and P venous
- blood flow drive by P arterial - P venous
What zone has highest blood flow in lungs?
zone 3
What is a physiologic shunt in lungs?
portion of CO diverted/ re-routed bypassing alveoli
What is bronchial blood flow? where does it drain?
blood supply to conducting airways, drains directly to LA
What does coronary blood flow drain? via what veins?
drains directly into LV through thebesian veins
What accounts for PO2 arterial being slightly less than PO2 alveolar?
physiologic shunt so some portion of blood bypasses alveoli and doesn’t get oxygenated and thus dilutes the oxygenated blood in the arteries
What 2 blood flows part of physiologic shunt?
- bronchial blood flow
- coronary blood flow
What is right to left shunt?
- direct flow of blood from right side of heart to left side of heart that never passes via alveoli/lungs so never gets oxygenated
What is hypoxemia?
low PO2 in arterial blood
Can hypoxemia be corrected by inhaling higher inspired O2?
No! shunted blood will dilute normally ox blood no matter how how alveolar PO2 is
What will higher fraction inspired O2 do to right left shunt?
- will increase dissolved O2 in capillary blood
- will not change total O2 content in blood b/c shunt is diluting it out
What happens to arterial PCO2 in right to left shunt?
stays the same
What do central chemoreceptors react to?
- sensitive to changes in arterial PCO2
- slight increase in arterial PCO2 –> increases ventilatory rate to get rid of extra CO2
When do chemoreceptors for O2 get activated? Are they more or less sensitive than chemoreceptors for CO2?
- less sensitive to changes in O2 than CO2
- O2 receptors activated at PO2 < 60 mmHg in arteries
What is left to right shunt?
- ox blood from left side of circulation enters right side
- oxygenated blood re-circulated through lungs
- pulmonary blood flow increases as part of left CO enters right side
What 3 things can cause left to right shunt?
ASD
VSD
Patent ductus arteriosus
What happens to pulmonary blood flow in left to right shunt?
pulm blood flow increases since part of left CO enters right side
What is PO2 on right side heart in left to right shunt?
- abnormally elevated PO2 in right heart
Does left to right shunt cause hypoxemia? right to left shunt?
- right to left causes hypoxemia
- left to right does not cause hypoxemia
What happens to Valveolar/Q in shunt vs dead space?
- lower V/Q in shunt
- higher V/Q in dead space
What is normal PO2 and PCO2 in alveoli?
PO2 = 100 PCO2 = 40
What normal PO2 and PCO2 in capillary?
PO2 = 40 PCO2 = 45
What is ventilation like in shunt vs deadspace? what about perfusion?
shunt: no ventilation, have perfusion so blood not getting oxygenated
dead space: no perfusion, have ventilation but its wasted because there is no blood to oxygenate
What is V/Q ratio in shunt?
V/Q ratio = 0 because there is no ventilation
What is V/Q ratio in dead space?
V/Q ratio = infinity because such little perfusion
What is alveolar PCO2 in case of dead space?
PCO2 = 0 in alveoli because there is no perfusion so no gas exchange so no CO2 to excrete
